1. Field of the Invention
The present invention relates to a process for producing optically active amines. In more particular, it relates to a process for producing an optically active amine represented by the formula (IV) ##STR5## wherein R.sub.7 and R.sub.8 each denote an alkyl group, aryl group or aralkyl group, providing that they do not denote the same group at the same time, and * indicates an asymmetric carbon atom.
2. Description of the Prior Art
The optically active amines represented by the formula (IV) are compounds important as resolving agents for preparing medicinal agents, agricultural chemicals, intermediates thereof, etc. It is already well known that they may be produced by first preparing a racemic compound and then resolving it by using an optically active acid or the like (cf., for example, Optical Resolution Procedures for Chemical Compounds, Vol. 1).
The prior method of producing an optically active amine by means of optical resolution comprises first preparing a racemic compound, then making an optically active acid or the like act thereon to form diastereomeric salts, crystallizing one of the diastereomeric salts thus formed by making use of the solubility difference of the diastereomeric salts, then separating the salt, thereafter reacting an alkali therewith to decompose the salt, and separating and recovering an optically active amine, one of the antipodes. Thus, the method has the disadvantages of complicated operations and poor efficiency.
The present inventors have made extensive study to solve the above-mentioned problems through a process using asymmetric synthesis. As a result, it was found and already proposed that an optically active amine can be prepared at a stretch by reacting an asymmetric reductant obtained from an optically active amine derivative and a boron hydride compound, with an oxime derivative and that an optically active amine of any desired absolute steric configuration can be prepared by proper use of either the anti-isomer or the syn-isomer of the oxime derivative [Japanese Patent Application KOKAI (Laid-Open) No. 63-99041; Tetrahedron Letters, 29, 223 (1988)].
Subsequently, the present inventors have made further study on a process that uses as the borohydride a metal borohydride which is easier to handle and less expensive. As the result, it has been found that by using a metal borohydride in combination with a specific mineral acid, the yield of the intended product can be improved and the amount of the metal borohydride to be used can be reduced. The present invention has been accomplished on the basis of the above finding along with further investigation.